The present invention relates to a sack trolley having a support frame presenting a lower pair of carrying wheels and upper handles and a hoist mechanism for climbing steps, which comprises a lifting arm equipped with return rollers at its free end and which can be swivelled about a swivelling axis parallel to the carrying wheel axis by means of a crank drive.
A similar sack trolley is disclosed in EP 0 533 650 B and its hoist mechanism for climbing steps has thoroughly proven itself, whereby the lifting arm designated as support frame engages in a crank guide solid with the support frame by way of a guide member in its longitudinal central region, and its end averted from the free end with the return roller is articulated on the crank arm of the crank drive. Because of the cooperation of support frame, crank drive and crank guide the support wheels describe a stilt movement reaching forwards or backwards with switching on, off and reversing of the drive motor, by means of which the support frame, is lifted as it travels up over the steps with the carrying wheels over the support wheels now resting on the ground and the support frame is transferred to the next highest step, or as it travels backwards the support wheels grip downwards on the next lowest step and as a result lower the support frame with the carrying wheels. This hoist mechanism thus requires considerable structural expense and is also associated with a corresponding space requirement which impairs the handiness of the sack trolley and undesirably increases its tare weight. Further to this, because the centre of gravity is misplaced upwards due to the hoist mechanism there is less stable handling of the trolley as it climbs steps, and not least of all in tricky situations the result is unsafe climbing assistance adapted minimally to the step profiles of the step due to the stilt movements of the support wheels.
The object of the invention is therefore to create a sack trolley of the type mentioned at the outset, which is distinguished by its easily handled and functional hoist mechanism with a simple and compact structure.
The invention solves this task in that the lifting arm is comprised of a swing guide component swivel-mounted about the swivel axis and a support component taking up the support rollers and longitudinally displaceable compared to the swivel guide component, whereby the crank drive is articulated on the support component with its crank arm. The free end of the support component is moved on the one hand by the superposition of the swivel movement of the swivel guide component effective thereon and the rotary movement of the crank arm on the other hand along an approximately reinform curved path, resulting in trouble-free climbing assistance according to the direction of movement with the support component and its support rollers. When the sack trolley is in a position corresponding to the travel position the support rollers of the support component are set engagingly on the next highest step tread for travelling up a step, are supported there and raise the trolley from one step to the other. On the contrary, during descent the support rollers remain supported on the respective upper step and the trolley is lowered by way of the lifting arm to the next lowest step, giving rise to a simple and functionally safe raising and lowering movement for automatic step travel. The lifting arm with the crank drive can be accommodated without difficulty in narrow spaces, for example between the carrying wheels of the sack trolley, and thereby alters the centre of gravity position of the sack trolley only insignificantly, in addition to which the hoist mechanism does not weigh excessively because of its simple structure and barely increases the tare weight of the trolley noticeably. The path of motion of the support rollers is influenced by appropriate adaptation of the crank throw, axle position, lifting arm length, articulation point of the crank throw on the support component and the like and is adapted to different step profiles, whereby the passage speeds of the path of the support rollers are adapted to the respective functional areas and the empty run and vertex areas are passed through more quickly than the lifting or lowering areas required for climbing steps.
A path of motion for the support rollers best adapted to normal use with climbing steps occurs when the crank axis lies in the direction of the support frame above the swivel axis, whereby the crank axis preferably lies above and the swivel axis lies below the swivel carrying wheel axis, and whereby the distance of the crank axis from-the support frame is preferably greater than that of the swivel axis, and the distance of the carrying wheel axis from the support frame is greater than that of the crank axis.
To facilitate assembly of the hoist mechanism and also to be able to subsequently alter the curved path for the support rollers the hoist mechanism plus the crank drive and lifting arm are designed as assembly unit and attached displaceably to the support frame, such that the assembly unit can be mounted problem-free with few hand grips and can be adjusted in place, leading to change in position of the swivel and crank axes and thus to relative displacement of the curved path of the support rollers.
It is particularly advantageous if the support rollers of the support component are equipped with a combined free-wheel and brake device which releases rotating of the support rollers in terms of the trolley travelling backwards, but which brakes counter-rotation of the support rollers. Release of the rotary movement of the support rollers in one direction and their braking instead of blocking in the counter-rotation direction facilitates climbing steps with the assistance of the hoist mechanism, since the position of the support rollers on the steps can be corrected effortlessly by the driver of the trolley without losing the necessary support effect of the support rollers.